This invention relates to a free-piston compressor, and more particularly to control of top clearance thereof.
A conventional free-piston compressor is as shown in FIG. 1. The compressor uses a refrigerant compressor used for a refrigerator and a free-piston stirling engine as driving means of compressor piston, and as a whole, comprises an engine portion, seal portion and compressor portion. First, the engine portion is explained. In container 1, working fluid of a stirling engine such as helium, nitrogen etc. is encapsulated. The working fluid (e.g., helium) flows through pipe 27 and is heated by heater 2, and cooled by cooler 3. Regenerator 4 is disposed between heater 2 and cooler 3. Heater 2, cooler 3 and regenerator 4 are disposed in pipe 27. The pipe 27 is also used for communicating compression space 22 and expansion space 23. A displacer 5 is slidably movable on an inner surface of container 1 in spaces 22 and 23. Rod 30 is connected to displacer 5 and disposed into a hole having gas spring space 24. Power piston 6 is also slidably movable on the inner surface of container 1 in space 28 and bounce space 26. The space 28 communicates with space 22 through path 29. A rod 7 is connected to power piston 6.
Next, compressor portion is explained. A the compressor piston 9 to which rod 7 is connected is slidably movable on an inner surface of a cylinder 8 in compression chamber 12 and spring chamber 25. There may be a gap between piston 9 and cylinder 8. The cylinder 8 is formed integrally with container 1. A low pressure pipe 10 and high pressure pipe 14 are connected to cylinder 8 through paths 31, 32, and in compression chamber 12 and paths 31, 32 of cylinder 8 proximate pipes 10 and 14, suction valve 11 and discharge valve 13 are disposed. A condenser 15 is inserted into high pressure pipe 14, and evaporator 17 is inserted into low pressure pipe 10, and these two pipes 14 and 10 are connected through expansion valve 16. A compression spring 18 is disposed in spring chamber 25 of cylinder 8 to prevent the occurrence of collision of compressor piston 9 and cylinder 8 due to difference of pressure between the working fluid (e.g., helium) and refrigerant upon cease of operation.
Next, the seal portion is explained. 19 denotes a path which communicates to the outside atmosphere. Sealing device 20 is disposed for preventing leakage of the working fluid to the outside atmosphere, and sealing device 21 is disposed for preventing leakage of refrigerant to the outside atmosphere.
Operation of the compressor is now explained. With a predetermined timing power piston 6 and displacer 5 move upwardly and downwardly, and as a result, a part of the heat which the working fluid (e.g., helium) obtains from heater 2 is converted to work against power piston 6. Normally, the phase angle of a position of displacer 5 is advanced by 60.degree.-90.degree. relative to the phase angle of a position of power piston 6.
As to the compressor portion, compressor piston 9, which is connected to rod 7 driven by the free-piston stirling engine, moves at the same speed as power piston 6. With upward and downward movement of compressor piston 9, gas refrigerant of low pressure and low temperature flows in low pressure pipe 10 through suction valve 11 into compression chamber 12 and is compressed therein to be of high pressure and high temperature, and then, is discharged through discharge valve 13 into high pressure pipe 14. Further, it flows into condenser 15 to become liquid phase with high pressure and then, flows through expansion valve 16 to become both gas and liquid phases of low pressure and low temperature and then, flows into evaporator 17 so that it is heated at evaporator 17 to thereby become gas phase of low pressure and low temperature, and then, flows into low pressure pipe 10.
At the above-stated one cycle process, some of the work that compressor piston 9 carried out on the refrigerant and the heat that the refrigerant obtained at evaporator 17 is discharged at condensor 15 through the refrigerant, and the cooled heat and the heated heat are used at evaporator 17 and condensor 15.
However, the above-stated conventional compressor has drawbacks which are: (i) compressor piston 9 may collide with suction valve 11 when compressor piston 9 moves upwardly since there is no means for controlling a top dead point of compressor piston 9, and (ii) volumetric efficiency and adiabatic efficiency decrease as a result of dead space increasing when compressor piston 9 moves too much downwardly.
The stroke of compressor piston 9 can be made constant by controlling, for example, heat input to heater 2. The refrigerant flows between spring chamber 25 and compression chamber 12 through a gap formed between compressor piston 19 and cylinder 8. However, the mass of refrigerant which flows in one direction differs from that of refrigerant which goes from so that a difference in average pressure occurrs between spring chamber 25 and compression chamber 12. Thereby, compressor piston 9 moves gradually downwardly or upwardly. This phenomenon is applicable to helium as working fluid which moves between bounce space 26 of the free-piston type stirling engine and working space (total space of compression space 22, cooler 3, regenerator 4, heater 2, expansion space 23, path 27, 29 and space 28) through gap formed between power piston 6 and container 1 so that compressor piston 9 moves gradually downwardly or upwardly due to the difference in the mass flowing in opposite directions.